Semiconductor lasers find applications in industrial processing, consumer, and medical fields. Some of these applications employ lasers in the mid IR (3 μm-8 μm) and long IR band (8 μm-15 μm). For example, atmospheric monitoring and trace-gas optical detection target molecular absorption bands throughout the mid-to-long IR range of the electromagnetic spectrum. Many biomedical applications rely on water's strong absorption of the mid-IR range. Likewise, many polymer-processing applications rely on strong absorption of energy in the mid-IR range. Military applications such as IRCM also typically employ mid-wave IR.
Semiconductor Quantum cascade (QC) lasers are suitable as compact sources emitting at wavelengths over 3 μm. While having different modes of operation, both QC and interband semiconductor lasers are often associated with highly divergent, yet diffraction limited beam emission along at least one optical axis. Depending on the architecture of the semiconductor laser, beam waist divergence angle and associated beam quality may be symmetric, or asymmetric characterized with a slow and fast optical axis. Therefore, compact commercial semiconductor lasers are often sold as a system or module equipped with integrated collimation optics. Collimation optics may include one or more lens or mirror. For asymmetric sources with poor beam quality in one direction, collimation optics typically includes at least a fast axis collimator (FAC) to reduce strong divergence in the “fast” direction. High power quantum cascade lasers with excellent beam quality in both axes utilize radially symmetric collimating lenses to provide complete collimation.
Prior to use, typical laser devices are collimated. The emitted laser radiation is typically characterized by a full-width at half-maximum of the far field beam intensity pattern in a plane perpendicular to the beam axis. FIG. 1A depicts a cross-sectional view of a conventional single transverse mode edge-emitting semiconductor laser module collimated by a fast axis collimator 101 if it is a broad area laser. Module 101 includes a semiconductor laser 110 disposed on a carrier or substrate 105. Plano-convex collimation lens 120 is substantially aligned with optical beam propagation axis 115. Plano-convex collimation lens 120 has two lensing surfaces, one being a plano lens surface 125 and the other being a convex (aspheric) lens surface 126. Adhesive 130 stands-off lens surface 125 a working distance from emission surface 121. Depending on beam quality asymmetry and associated divergence angles, plano-convex collimation lens 120 may have rotational symmetry about the optical axis or translational symmetry along the slow axis and rotational symmetry along the fast axis. FIG. 1B depicts an isometric view of a conventional laser module 102 including a rotationally symmetric collimation lens 120A in which plano lens surface 125A is circular and convex lens surface 126A is associated with some asphericity. FIG. 1C depicts an isometric view of a conventional laser module 103 including a translationally symmetric collimation lens 120B in which plano lens surface 125 is rectangular and convex lens surface 126B is substantially cylindrical with a fixed or varying radius curvature about a longitudinal axis 116 aligned with a slow optical axis.
One issue with module 101 is that a high angle of incidence on the plano surface can lead to significant optical loss. Another issue with module 101 is that it can be difficult to properly set a correct centration and working distance between the lens and an emission surface as it relies as six-axis alignment while adhesive 130 is cured. Improvements in optical efficiency, and reductions in manufacturing complexity/expense are therefore advantageous.